Hi Sana,
@chhantyal's workaround ought to work in any place you have a Document object, including inside a StreamField - can you give more details about why you think this won't work in your case?
(Is it because you're just outputting the StreamField in one go with {{ page.body }}, and never referring to the document directly in the template? If so, see
http://docs.wagtail.io/en/v1.5/topics/streamfield.html#template-rendering for details of how to customise the rendering on a per-block basis.)
Cheers,
- Matt